This invention relates generally to the field of paste extrusion of low density polytetrofluoroethylene to form elongated articles such as hollow tubes and insulated conductors. More particularly, it relates to an improved method for forming such articles which will substantially increase the speed of production, and thus lower the cost of manufacture without sacrificing quality.
The process of extruding polytetrofluoroethylene by paste forming of dispersion polymerised resin is well known commercially. In this process, the extrudate is formed of powdered polytetrofluoroethylene material with an extrusion aid forming 10 to 30 percent of the volume of the extrudate and passed through an extrusion die forming part of a ram extruder to form a hollow tube or filament, tape or other shaped article. The hollow tube may be extruded about a continuous length of wire which passes through a guide tube, the outer end of which is positioned in the land of the die, thus forming an insulated conductor. A Harlow U.S. Pat. No. 4,529,564, granted July 16, 1985 discloses a variation of this process in which the extrudate is passed through driven nip rollers to regulate the speed of the extrudate with the conductor moving at a fixed speed, and the extrudate initially moving at a slower speed prior to stretching, but moving at the same speed once the stretching operation has been accomplished. In the Harlow patent, the tube is heated to a first degree for extrusion aid removal, to a higher degree during stretching, and finally to a still higher degree for sintering.
As suggested in the Okita U.S. Pat. No. 4,225,547, it is possible to manipulate the extrudate before removal of the extrusion aid, without the addition of heat beyond that provided by the extrusion die. More importantly, it has an appreciable strength in the extrusion direction, as a result of being passed through the extrusion die. Okita, however, did not appreciate that a volumetric expansion could be effected by stretching. The Okita patent, however, teaches how to adjust wall thickness and diameter of the tubing without effect on density.
In the Harlow Patent, the product is produced on a continuous basis, but it is apparently unappreciated that the polytetrofluoroethylene can be stretched before removing the extrusion aid and without additional heat. Because of this, the Harlow method requires three separate heated zones, the first of which is employed solely to remove the extrusion aid, the second of which is used to elevate the temperature of the now dried extrudate to promote stretching, and, finally, a third zone is used to sinter the product in stretched condition.